Studies of the dielectric constant in solid H2,4He,and Ne

Measurements of the static dielectric constant and of the resulting polarizability _CM defined by the Clausius-Mosotti relation are reported for several molecular solids under conditions of quasiconstant density. The range of measurements was between 0.4 and about 15 K, and pressures up to about 1 kbar were used. In solid H₂, the changes of as a function of ortho concentration are reported at 4.2 and 12 K, from which it is concluded that [(ortho)–(para)]/0.005, which is approximately 2 1/2 times larger than in the liquid phase. Furthermore the temperature change of _CM is studied for various densities at comparable ortho concentrationsX. Particular attention is paid to the orientational order-disorder transition in solid H₂ withX0.65, where _CM shows an abrupt change that appears to be a function ofX, density, and thermal history. In general _CM is lower in the ordered (cubic) phase than in the disordered (hexagonal) phase. The temperature change of _CM in almost pure para-H₂, measured between 4 and 0.4 K, is presented and is compared with other recently reported experiments. It is concluded that the mechanism that produces the change of _CM withT and at the order-disorder phase transition is not understood. In solid Ne, measurements of _CM are reported for one sample assumed to be at zero pressure and one where the density is quasiconstant. There is a strong temperature dependence in _CM for the former above 5 K (in agreement with previous findings), while for the latter _CM appears to be constant within the uncertainties introduced by strain-induced hysteresis effects. The sample of solid⁴He, investigated between its melting point at 4.3 and 0.8 K at constant density, shows a very weak temperature dependence of _CM, which is also consistent with previously reported measurements.Research supported by a grant of the U.S. Army Research Office (AROD) and from a contract with the Office of Naval Research. A preliminary account of this research was given inBull. Am. Phys. Soc.20, 672 (1975).